Let's say, for example, that there are only 5 kinds of movies: movies for young children, movies for tweens, movies for Highschool kids, light movies for adults, and graphic movies for adults.
When there was G, PG, and R, movies could fit in 2/2/1 and with plenty of line blurring. PG encompassed a large chunk of what mature human beings wanted to see. Good. Studios had no hesitation releasing a PG.
Then there was G, PG, 13, R, and NC-17. It almost lines up perfectly with the five types from before, but it is too weighted for the younger side. 13 isn't a perfect fit for high schoolers. Not good.
And then the middle, teenager rating was forced into becoming the "for everyone" rating. That way a single movie would be appropriate for an entire family to watch together. With no wiggle room.
They also took away a lot of wiggle room for R by becoming more strict with IDs in the last 10 years.
15 would be built around the idea of adding more wiggle room to "movies for everyone." You wouldn't even have to call it 15 or 16. Could be A for adolescent or YA for young adult.
You'd end up with:
Movies For Everyone With a Low Chance of Upsetting Kids. (PG-13)
And
Movies For Everyone With a Slightly Higher Chance of Upsetting Your Kids, But Will Make Dad Happier And Usually Be Fine If The Kid Is Like 14. (New rating)